The present invention relates to a method by which to fashion wheels for motor vehicles, in particular cast wheels, and to the wheels obtainable by such a method. More exactly, the invention relates to a method of manufacturing a wheel which when fitted with a pneumatic tire will be capable of ensuring a reduction in the forces of inertia generated by the assembled wheel and tire once set in rotation. It is known that a rigid body such as a mechanical component, when rotated about a fixed axis, becomes invested with centrifugal force and torque. In the specific instance of road wheels, fitted with tires and secured to the relative supporting members of a motor vehicle, the inertial and torque forces generated in rotation give rise to serious functional disturbances; their effect is to trigger rotational reactions in the supporting members, whereupon vibrations are set up and transmitted through the mechanical components to which the selfsame supporting members are mounted. What is more, the vibrations generated in this manner are clearly discernible through the steering system of the vehicle and, at higher road speeds, liable not only to cause discomfort to the driver but also to create difficulties in maintaining direction, with the result that normal road-holding is adversely affected.
The variations in inertial forces generated in this manner can be measured utilizing special equipment designed to give a reading from polar coordinates centered on the axis of rotation. In particular the operator can measure variations in inertial forces referred to the tire, exclusively, and to the wheel exclusively, also to the wheel and tire together. The problem in question has long been recognized, as witnessed by U.S. Pat. No. 1,860,216 which explicitly discloses the notion of securing a sleeve to the wheel disc, disposed with its axis offset from the axis of rotation and connected thus to the hub of the vehicle. The effect is to generate a greater weight in one half of the wheel than in the other, the heavier half being that occupied by the valve hole. The degree of offset between the axis of the sleeve and the axis of rotation of the assembled wheel and tire is established by determining the extent to which the selected tire registers out of balance. Thereafter, the unbalanced tire is fitted to the unbalanced wheel in such a manner that each imbalance can compensate the other. In short, the localized masses upsetting the balance respectively of the tire and of the wheel will be disposed on opposite sides of the axis of rotation to the end of achieving a mutual compensation.
Mention is made as early as 11-15.sup.th Jan. 1971, in a publication by S.A.E, the American Society of Automotive Engineers, of the concept of reducing, or rather compensating variations in kinetic forces by fitting pneumatic tires to wheels eccentrically. By contrast, the prior art embraces the technique, as stated in the background of U.S. Pat. No. 3,808,660 for example, of measuring the variations in inertial forces generated by a tire and removing portions of material (rubber) in the event that such variations exceed a prescribed limit, effecting a series of measurements and successive removals of material. In order to avoid effecting numerous measurements and successive removals of material, while ensuring that a balanced tire will not ultimately form part of an unbalanced wheel-tire assembly, U.S. Pat. No. 3,808,660 discloses the procedure of fitting the tire to the wheel, effecting a measurement as described above and localizing the point at which peak amplitude of the fundamental frequency occurs, then offsetting the center hole from the axis of rotation by an amount commensurate with the peak value measured. The offset and the heavy portion of the tire are located on opposite sides of the axis of rotation in such a way that the one is cancelled out by the other. It will be seen from the drawings of the patent in question that the offset is obtained by the addition of a ring, inserted into the flanged center hole of the wheel disc and fitted over the hub pivot. The bore and the surface of revolution of the ring are offset by a predetermined amount, which is a function of the peak value as measured for the assembled wheel and tire. When mounting the wheel to the hub, the ring can be rotated on the pivot to find the correct angular position of the offset in relation to that of the pivot; in like manner, the ring can be repositioned or replaced in the event that the initial correction should prove inaccurate at a subsequent verification.
Referring again to the prior art, U.S. Pat. No. 3,951,563 addresses the difficulty in achieving a faultless assembly of the disc and rim of a wheel, with the center hole of the disc and the axis of rotation coinciding to best advantage. The method disclosed is one of setting the assembled wheel in rotation about a reference axis, measuring the variations in the radial forces generated during rotation, then locating the center hole in a position, offset from the axis of rotation, at which minimum variation occurs.
Similarly, there are dynamic tests used by motor vehicle manufacturers for measuring the high and/or the low value of the fundamental frequency produced by radial forces of inertia, both in wheels and in tires, and identifying the corresponding angular position. Having run the tests, the tire is fitted to the wheel such that the position identified as `high` for the one coincides with the position identified as `low` for the other, a method known as "match mounting" to those skilled in the art. Such a procedure is somewhat lengthy and costly, however, especially when considering that it has to be repeated on each wheel and tire assembled. Given that each motor vehicle has at least four wheels, it requires little imagination to envisage the annual cost of match-mounting wheels and tires for all the vehicles produced in any one year. The ongoing demand for improvement in this field has been met on the one hand by an effort on the part of tire manufacturers to provide increasingly better balanced products, or at least to provide tires bearing an indication of the area of maximum or minimum imbalance, i.e. the position reflecting the high or low value of the fundamental frequency generated in rotation by radial forces of inertia. On the other hand, the makers of wheels have made similar efforts with their products so that discs also bear these same `high` and `low` indications. Accordingly, it becomes possible for wheels and tires to be assembled correctly without any dynamic balancing operations being necessary, but simply by positioning the `high` area of the tire to coincide with the `low` area of the respective wheel. Given the need to provide wheels with a `high` or `low` reference mark as described above, and with the end in view of reducing the offset between the center hole and the circle described by the axes of the stud holes, by way of which the disc is secured to the hub of a vehicle, U.S. Pat. No. 4,279,287 discloses a method of locating the `high` or `low` reference at a predetermined angular position of the wheel, in this instance a pressed steel disc. This patent method envisages the preparation of a rim and disc assembly, the disc being inserted into and welded to the rim, of which the disc affords at least one center hole disposed offset from the axes of the circumferential seatings destined to receive the beads of the tire. In practical terms the procedure is one of punching the center hole and the stud holes in a single operation, and in such a way that the axis of the former is concentric with the axis of the circle described by the latter, but offset in relation to the axis of the disc-rim assembly overall. The axis of rotation of the wheel is thus displaced by translation away from the axes of both circumferential seatings, and the imbalance built into the wheel is a static imbalance.
Examining these references in detail, it is clear that all relate to the pressed steel disc type of wheel, and therefore to a production technique that is relatively imprecise, not least by reason of the dimensions typically involved.
In every instance, moreover, it emerges that the preferred method pursued is one of offsetting the center hole and the circle of stud holes from the circumferential seatings destined to accommodate the tire beads, which obviously are coaxial, such that the eccentricity of the axis of rotation is taken up by the elasticity of the pneumatic tire; needless to say, considerable vibration would be transmitted to the vehicle as a result of any such eccentricity if the tire were rigid.
The object of the present invention is to afford a method of fashioning alloy wheels, of the type cast in moulds and then machined to remove an allowance of material, whereby an "imbalance" is incorporated of which the effect, in each wheel fitted with a respective pneumatic tire and secured to the hub of a motor vehicle, is to achieve an improved overall balance both of the assembled wheel and tire and in the moving mechanical parts of the vehicle with which the wheel is associated.
A further object of the invention is to produce alloy wheels with a deliberate imbalance by employing machine tools of conventional design rather than the special toolage currently required as in the case, for example, of the patent methods referred to above.